Microwave tube using electronically tunable cavity resonator

ABSTRACT

A microwave cavity resonator, such as the circular electric mode resonator of a coaxial magnetron, is electronically tuned by coupling an electronically tunable array of reactive loading elements to the resonator. Tuning means, such as PIN diodes or ferrite tuning members, are associated with respective ones of the reactive loading elements for varying the reactive loading effect of respective ones of said reactive loading elements on the composite resonator for tuning the resonant frequency of the composite resonator in accordance with the reactive loading effect of the elements. In a preferred embodiment, the reactive loading structure is preferably disposed inside of the resonator. In the case of a circular electric mode resonator, the individual reactive loading elements are preferably energized in a symmetrical manner relative to the resonator to avoid converting energy from the circular electric mode to certain other unwanted lower Q modes.

United States Patent [1 1 Farney MICROWAVE TUBE USING ELECTRONICALLY TUNABLE CAVITY RESONATOR [75] Inventor: George K. Farney, Boxford, Mass.

[73] Assignee: Varian Associates, Palo Alto, Calif.

[22] Filed: Mar. 22, 1974 [21] Appl. No.: 453,667

[52] US. Cl. 315/3955; 315/3959; 315/3977; 331/90; 333/83 [51] Int. Cl. H01J 25/50 [58] Field of Search 315/3955, 39.57, 39.59,

[56] References Cited UNITED STATES PATENTS 2,752,495 6/1956 Kroger 315/3955 3,048,797 8/1962 Linder 315/3951 X 3,094,664 6/1963 Kibler... 331/96 3,163,835 12/1964 Scott 333/83 R 3,171,086 2/1965 Gerlach 330/43 3,333,148 7/1967 Buck 315/3977 3,334,267 8/1967 Plumridge.... 315/3355 3,400,298 12/1965 Krahn i 315/3973 3,521,194 7/1970 Lowe 333/83 R X 3,568,110 3/1971 Ivanek 333/83 3,573,540 4/1971 Osepchuk 330/43 Dec. 16, 1975 Dench 333/83 Karp 333/83 R [57] ABSTRACT A microwave cavity resonator, such as the circular electric mode resonator of a coaxial magnetron, is electronically tuned by coupling an electronically tunable array of reactive loading elements to the resonator. Tuning means, such as PIN diodes or ferrite tuning members, are associated with respective ones of the reactive loading elements for varying the reactive loading effect of respective ones of said reactive loading elements on the composite resonator for tuning the resonant frequency of the composite resonator in accordance with the reactive loading effect of the elements. In a preferred embodiment, the reactive loading structure is preferably disposed inside of the resonator. In the case of a circular electric mode resonator, the individual reactive loading elements are preferably energized in a symmetrical manner relative to the resonator to avoid converting energy from the circular electric mode to certain other unwanted lower Q modes.

15 Claims, 14 Drawing Figures ELECTRONIC TUNER RECEIVER L R. F.

[0 ANTENNA US. Patent Dec. 16,1975 Sheet 1 012 3,927,347

25 %%Q' RECEIVER Sheet 2 of 2 3,927,347

US. Patent Dec. 16 1975 FIG. 9

FIG. I4 sum REGISTER IOOOOOOQOOOOOO WHllW-WH MICROWAVE TUBE USING ELECTRONICALLY TUNABLE CAVITY RESONATOR BACKGROUND OF THE INVENTION The present invention relates in general to electronically tunable cavity resonators and to microwave'tubes using same, such as circular electric mode'magnetrons.

DESCRIPTION OF THE PRIOR ART Heretofore, it has been proposed to electronically tune a circular electric mode magnetron by coupling the circular electric mode cavity, employed for stabilizing the operating frequency of the magnetron, to a ferrite phase shifter disposed in an appendage affixed to the body of the tube. By electronically varying the phase of energy reflected to the circular electn'c mode resonator, the frequency of the magnetron could be shifted or tuned. A solenoid was wound around the ferrite member of the phase shifter for varying the magnetic field in the ferrite to produce the electronically variable phase shift.

It was anticipated, by the proponents of this scheme, that such an arrangement could produce between 1% and 2% of electronic tuning. However, the structure is relatively large having overall dimensions generally comparable to the size of the tube at Ku-band. The aforecited tube is presently under development with the Department of the Navy under NAVELEX, Contract No. N00039-73-C- 0080 and is described in the first and second Quarterly Reports under that contract. These reports have Document numbers PT-3938 and PT-4062, respectively. The second quarterly report is dated Dec. 13, 1973.

SUMMARY OF THE PRESENT INVENTION The principal object of the present invention is the provision of an electronically tunable cavity resonator and microwave tubes using same.

In one feature of the present invention, a plurality of reactive loading elements are coupled in electromagnetic field exchanging relation with the excited resonant fields of a cavity resonator for reactively loading the resonator to define a composite reactively loaded cavity resonator structure. Means are provided for varying the reactive loading effect of respective ones of said reactive loading elements on the composite structure for tuning the resonant frequency of the composite cavity resonator, whereby the resonator is tuned electronically.

In another feature of the present invention, the reactive loading elements are disposed within the cavity resonator, whereby improved coupling to the resonator is obtained and whereby the size of the composite resonator structure is minimized.

In another feature of the present invention, the reactive loading structure comprises an array of reactive loading elements with electronic tuning means associated with each element for electronically varying the reactive loading on the composite resonant structure.

In another feature of the present invention, tuning means are associated with each tuning element of the array, such tuning means including one or more diode means for electrically shorting the. fields of respective ones of the reactive loading elements in accordance with an electrical signal applied across the respective diode means.

In another feature of the present invention, the tuning means associated with each of the reactive loading elements comprises a ferromagnetic means for varying the inductive reactance of respective ones of the reactive loading elements in accordance with a magnetic signal applied to the ferromagnetic means.

In another feature of the present invention, the reactive loading structure comprises a segmented conductive ring with adjacent segments of the ring being electrically connected together via diode means, whereby electrical bias signals applied to the diode means, for rendering the diode means conductive, changes the effective conductive length of the ring coupled to the resonator.

In another feature of the present invention, the reactive loading structure comprises an array of reactive loading vanes projecting into the resonator from a wall thereof.

In another feature of the present invention, the circular electric mode stabilizing cavity of a coaxial magnetron is tuned by a plurality of reactive loading elements coupled in energy exchanging relation with the circular electric mode of the circular electric mode resonator, the reactive loading of respective ones of the elements being varied by means of diode means or ferromagnetic means associated therewith to obtain electronic tuning of the operating frequency of the magnetron.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a transverse schematic sectional view, partly in block diagram form, of a circular electric mode magnetron incorporating features of the present invention,

FIG. 2 is a sectional view of the structure of FIG. 1 taken along line 22 in the direction of the arrows,

FIG. 3 is an enlarged detail view of a portion of the structure of FIG. 1 delineated by line 33 and also depicting, in block diagram form, the modulator for electronically varying the tuning,

FIG. 4 is an enlarged detail view of a portion of the structure of FIG. 3 delineated by line 4-4,

FIG. 5 is a view similar to that of FIG. 2 depicting an alternative arrangement of the electronic tuning means within the circular electric mode cavity of the tube,

FIG. 6 is an enlarged detail view of a portion of the structure of FIG. 5 delineated by line 6-6 and depicting an alternative tuning arrangement,

FIG. 7 is a sectional view of the structure of FIG. 6 taken along line 7-7 in the direction of the arrows,

FIG. 8 is a plot of electric field strength E of the circular electric mode versus frequency f depicting the frequency change associated with activation of one of the diode elements of FIGS. 6 and 7, as a function of the current through the diodes,

FIG. 9 is a plot of series resistance R; versus current for one of the pair of diode elements in the tuning scheme of FIGS. 6 and 7,

FIG. 10 is a plot of current I versus voltage V through one of the tuning diodes of the tuning scheme of FIGS. 6 and 7,

FIG. 11 is an enlarged detail view similar to that of FIG. 3 depicting an alternative tuning scheme of the present invention,

FIG. 12 is a sectional view of the structure of FIG. 11"

taken along line 12-12 in the direction of the arrows,

FIG. 13 is a schematic line diagram depicting the plan view of a circular electric mode cavity showing a preferred symmetrical sequence for energization of the tuning elements therein, and

FIG. 14 is a schematic circuit diagram of a shift register as employed in a tuning modulator as used in the tuning scheme of FIGS. 1-4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIGS. 14, there is shown a circular electric mode magnetron oscillator incorporating features of the present invention. The magnetron 11 includes a central cylindrical thermionic cathode emitter 12 coaxially surrounded by a circular array of vane resonators 13 operating at anode potential. Alternate vane resonators 13 are coupled via coupling slots in the rear wall thereof to a circular electric mode resonator 14 coaxially surrounding the array of anode resonators 13.

The circular electric mode cavity 14 is dimensioned and arranged for supporting the TE mode at the operating frequency of the magnetron oscillator 11 for stabilizing the operating frequency thereof. RF power output is coupled from the circular electric mode resonator 14 via a coupling iris 15 and thence via waveguide 16 and microwave window 17 to a suitable microwave load, such as an antenna, not shown. A magnet, not shown, produces an axial magnetic field in the annular space between the cathode 12 and the surrounding array of anode resonators 13. Under the influence of the crossed electric and magnetic fields between the cathode and anode, electrons are caused to circulate around the cathode 12 and to form into spokes of space charge, in the conventional magnetron mode of operation, for generating microwave energy on the anode circuit 13 and in the circular electric mode cavity 14 coupled thereto.

An electronic tuner, of the present invention, is incorporated in the magnetron structure 1 1 for electronically tuning the operating frequency of the magnetron.

The electronic tuner includes a reactive loading structure 18 disposed within the circular electric mode cavity in electromagnetic field exchanging relation with the circular electric mode for tuning the cavity and, thus, the operating frequency of the magnetron 1 1. The reactive loading structure 18 is shown in FIG. 3 and comprises an array of radially directed vanes 19 projecting from the outer wall of the circular electric mode resonator 14 toward the central anode structure. In a typical example, the vanes 19 have a height h measured from their root portion to their tips of approximately one-tenth of a wavelength. A pair of back-to-back mounted PIN diodes 21 are mounted between adjacent vanes 19 at a point for example midway between the root portions and the tip portions thereof.

A wire lead 22 is brought to the midpoint connection of each of the diodes 21. The lead extends generally perpendicular to the electric field vector of the circular electric mode such as to provide minimum perturbation thereof. The leads 22 pass out through feedthrough insulators 23 to respective output terminals n,-n,, of a tuning PIN modulator 24. In FIG. 1, the tuning PIN modulator is designated as the electronic tuner 24. The tuning PIN modulator 24 serves to provide a bias voltage to the back-to-back connected diodes 21 which switches between a first and second state. In a first state the diodes 21 are back-biased so as to be rendered nonconductive such that the respective diodes represent an open circuit between adjacent vanes 19. In the second state, the output of the tuning modulator 24, as applied to a respective pair of diodes 21, is such as to forward-bias the diodes 21 and to render them conductive. In this state the diodes appear as a short circuit between adjacent vanes 19.

In a typical example at S-band, the circular electric mode resonator 14 has an axial height of 3.10 inches, an inside diameter of 2.98 inches, and an outside diameter of 8.88 inches. 250 inwardly directed vanes 19 define 250 slot-like regions between adjacent vanes 19 containing the back-to-back connected PIN diodes. The vanes 19 have thicknesses t in the direction of the electric vector (circumferential) of the resonator which are approximately equal to the thickness of the space or slot between adjacent vanes. In addition, the width w of the adjacent vanes 19 in a direction normal to the electric field vector of the circular electric mode resonator is 0.200 inch and preferably less than the 0.400 inch height h of the respective vanes 19 which is about 10% of the height of the cavity 14. Each of the vanes or regions between adjacent vanes serves to provide a certain inductive reactive loading on the resonator 14 when the diodes 21 are open circuited or nonconductive. As the diodes 21 are rendered conductive in a respective slot, the magnetic field of the TE mode which was coupled into the slot or region between adjacent vanes is excluded, thereby changing the effective volume of the resonator 14 and, thus, its resonant frequency. Thus the tube 11 is tuned in discrete frequency increments over a certain tuning range, as of one to several percent, by selectively rendering conductive respective pairs of diodes 21 in the respective slots of the array of coupled reactive members 18.

The electronically tuned circular electric mode resonator 14 may also be employed to advantage as an electronically tuned filter for rejecting signal and noise falling outside of the passband of the magnetron 11. More particularly, in the case of an electronically tuned radar, the magnetron output transmitter tube 11 is electronically tuned to any one of a number of desired frequencies within a tunable frequency range as determined by the output of the electronic tuner 24. It is desired that the receiver 25 which is to receive the echo pulses be tuned to the same frequency as the transmitter. Accordingly, the receiver 25 is coupled to the circular electric mode cavity resonator 14 via the intermediary of a TR tube 26 and coupling iris 27.

Radar ccho pulses received on the antenna are propagated into the coaxial resonator 14 via the microwave window 17, waveguide 16, and iris 15. Since the resonator 14 has been electronically tuned to the frequency of the transmitted pulse it is at the proper frequency to receive the echo pulse and reject all other signal and noise. The signal then passes through the cavity to the receiver via the TR tube 26. The TR tube effectively isolates and protects the receiver 25 during the duration of the transmitted pulse of the magnetron 11. Thus, in the system of FIG. 1, the input to the receiver 25 is automatically tuned to track the frequency of the transmitted pulse.

In addition to the electronic tuning provided by the coupled array of reactive loading members 18, conventional mechanical ring tuners may be placed within the circular electric mode resonator 14 in the conventionai manner as disclosed in US. Pat. No. 3,441,795. Although the PIN diodes 21 of FIG. 4 have been shown with the N regions connected to the common wire lead 22, this is not a requirement. As an alternative, the diodes 21 may be arranged with the P regions connected to the common lead 22 and the N regions connected to the adjacent vanes 19. Also the PIN diodes need not be packaged but may comprise only the chips or dies which are mounted between the vanes 19 without separate packages. 7

Referring now to FIG. 5, there is shown a view similar to that of FIG. 2 showing an alternative position for the reactive loading array 18. In this example, the reactive loading vanes 19 project axially of the cavity 14 from the top and/or bottom annular walls 26. The array 18 is preferably positioned at the radial center plane of the resonator 14 where the electric field has a maximum intensity for maximum coupling to the fields of the resonator 14. In this position, the reactive loading array 18 has less effect on the output coupling iris and associated components, as this output coupling is typically achieved in the outer side wall of the resonator 14.

Referring now to FIGS. 6 and 7, there is shown an alternative reactive loading array 28 incorporating features of the present invention. The reactive array 28 comprises a segmented conductive ring structure supported from the wall of the resonator via a low loss insulator body 29, as of alumina or beryllia. Adjacent segments 31 are connected together via the intermediary of PIN diodes 32. Alternate ring segments 31' are conductively connected to RF ground, i.e., the wall of the resonator 14, via conductive leads 33. The other ring segments 31 are connected to the output of the tuning PIN modulator 24 via leads 34 and feedthrough insulators 35. The diodes 32 are connected back-toback relative to respective leads 34 and corresponding ring segment 31 such that upon energization of a respective lead 34 with a bias potential that forwardbiases the diode 32, the given ring segment 31 is electrically connected in circuit with the adjacent grounded ring segment 31 Thus, when all of the diodes 31 are in the off condition the tuning ring is electrically segmented and has a minimum displacement effect of the electric field within the resonator. However, upon rendering all of the diodes conductive, the segmented ring becomes a continuous tuning ring within the resonator and has a maximum tuning effect by effectively reducing the volume of the resonator by that part directly affected by the tuning ring 28. The resonator 14 is tunable in discrete frequency increments between the two ends of the tuning range by selectively energizing respective segments of the composite tuning ring structure. As in the previous embodiments, the tuning ring 31 and 31 may be placed adjacent the top or the bottom end walls of the resonator or along the outer side wall of the resonator 14.

Referring now to FIGS. 8-10 there is depicted the electrical tuning effect obtained by energizing one of the reactive loading elements of the tuning structure of the type employing the diodes 21 and depicted in FIGS. 14 and FIGS. 6 and 7. More particularly, as shown in FIG. 8, the output RF electric field signal, with no diodes rendered conductive, will have a certain frequency f and a certain signal amplitude A As one of the diodes is turned on or rendered conductive, as shown in FIGS. 9 and 10, the series resistance R, of the diode begins to decrease with increasing current I. The

current increases with increasing forward bias voltage V after a certain turn on or threshold voltage has been applied to the diode 21. Thus, as the diode 21 is switched from the off state, corresponding to frequencyf in FIG. 8, to the final condition of being fully conductive and thus having a minimum series resistance R, as indicated for the output centered at f 1 the frequency shifts in a continuous manner with increasing current I through the diode 21. Also the RF losses increase in the respective diode during the turn on process, thus lowering the Q of the composite resonant structure as indicated by the dotted signal curve 38 having decreasing amplitudes A A and A respectively.

Thus, in some cases, it may be desirable to obtain the continuous tuning spectrum between the first and second frequencies f and f corresponding to the fre quency shift obtained by fully energizing one of the reactive loading elements of the tuning array. However, as can be seen from FIG. 8 there is a corresponding reduction in Q for the intermediate values of conduction of the respective diodes. In some cases, this decrease in Q may be tolerable to obtain the continuous frequency shift. However, in many instances it may be desirable to avoid this Q loading effect and therefore to rapidly switch the diode from a nonconductive state to the fully conductive state as indicated by the RF electric field signals corresponding to frequency f and f respectively.

Referring now to FIG. 11 there is shown an alterna tive embodiment of the reactive loading structure 18. In the embodiment of FIGS. 11 and 12 the diodes 21 between adjacent vanes 19 have been replaced by,

ferrite members 41 which are disposed adjacent the side wall of the circular electric mode resonator at the root portions of the vanes 19'. The vanes 19' are also modified to include a magnetically permeable structure 42, as of soft iron, which projects through the side wall 43 of the resonator 14 to an external circumferentially directed yoke portion 44 which interconnects adjacent pole piece portions 42 of the vanes 19'. Windings 45 are wound on the yoke portions 44. The windings are either selectively energized for polarizing the pole piece portions 42 to provide a magnetic field through the ferrite 41 or the solenoidal windings 45 may be connected together in series and energized together. The inductive loading effect of the individual vane structures 19' on the operating mode of the circular electric mode resonator 14 is a function of the magnetic field strength in the ferrite members 41.

As in the embodiments of FIGS. 1-5, the ferrite tuner structure of FIGS. 11 and 12 may be incorporated either in the bottom, top, or side wall of the circular electric mode resonator 14 for electronic tuning thereof. The electronic tuner 24 may have individual outputs connected to respective ones of the windings 45 or the electronic tuner 24 may have one output which is applied to all of the series connected solenoidal windings 45. Suitable ferrite materials include yttrium gadolinium iron garnet doped with between 0 and 15% dysprosium.

Referring now to FIG. 13 there is shown a preferred spacial sequence for activation of the individual tuning elements wherein as much symmetry as possible is maintained in the electric field perturbation within the circular electric mode resonator 14. More specifically, if the individual tuning elements are separately activated, activation of a first element is followed by activation of a second element in a diametrically opposed position with the next pair of elements being activated to bisect the included angle between previously activated pairs of elements. A sequence of individual activation and the spacial distribution is shown in FIG. 13 where the numbers correspond to their sequential position in the sequence of activation. This symmetrical method of activating the individual tuning elements decreases the amount of asymmetry in the perturbed electric fields of the resonator 14, thus reducing the amount of mode conversion from the desired TE mode to certain other undesired modes having lower Q.

Referring now to FIG. 14 there is shown a shift register 48 as employed for example in the electronic tuner 24 for deriving the outputs to the individual tuning elements, such as the diodes or individual windings 45. The output leads 49 from the shift register, after suitable amplification, by amplifiers, not shown, are connected for the preferred sequence of activation as indicated in FIG. 13. That is, the first lead n is connected to the first tuning element to be activated, as indicated by l in FIG. 13, and so forth. The PIN diodes 21 need not be packaged but may merely be in chip form mounted between the adjacent vanes 19 as shown in FIG. 4.

While the circular electric mode magnetron of FIGS. 14 has been shown with the external circular electric mode cavity 14 being evacuated, this is not a requirement and in some embodiments a conventional gas tight wave permeable envelope, such as a ceramic cylinder, is positioned inside of the resonator 14 at the outside of the inner wall such that the resonator 14 is not evacuated. Also, the invention is applicable to other types of circular electric mode magnetrons such as the type exemplified by US. Pat. No. 3,231,781 issued Jan. 25, 1966, wherein the circular electric mode resonator which is employed for stabilizing the frequency of the tube is coaxially mounted on the centervline of the tube and is surrounded by the circular array of vane resonators, which face outwardly toward the surrounding cathode emitter. In this latter case, the array of tuning elements 18 is preferably located on one of the end walls of the circular electric mode resonator so as not to interfere with the system of coupling slots in the outer side wall of the circular electric mode resonator.

The advantage of the electronic tuning arrangements of the present invention are that they provide a means for very rapidly tuning the frequency of a cavity resonator or magnetron to certain predetermined frequencies which can be precisely predetermined in accordance with the output of the electronic tuning modulator 24. Furthermore, the frequency need not be swept in accordance with any linear sweep function but may be randomly positioned anywhere within the electronic tuning range of the tube by causing the output of the tuning modulator 24 to have the desired random sequence. The output of the tuning modulator 24 may also be employed for tuning the frequency of the local oscillator or receiver to precisely the same predetermined frequencies as that of the magnetron by merely utilizing the tuning outputs of the modulator 24 for exciting a suitable tuning arrangement in the receiver.

Thus, the electronically tuned magnetron avoids the problems of the prior art mechanical tuners in that the frequencies can be more precisely determined and the frequency changed much faster and in any desired manner within the tunable band without moving parts.

This electronically tuned magnetron is particularly useful for electronic counter measures, dithering the target enhancement, and for chirped radars. In the latter instance, the frequency of the transmitted microwave pulse is varied over the duration of the pulse in such a manner as to be an inverse function of a frequency dependent delay in the receiver to derive pulse compression within the receiver for improved signal-tonoise ratio.

As an alternative to use of PIN diodes for use in the embodiments of FIGS. 1-10, such diodes 21 and 32 may be replaced by ferroelectric ceramic members, such as barium titanate comprising 73% Ba Ti 0 and 27% Sr Ti 0 In such a case, a suitable electrical bias across the ferroelectric ceramic produces a substantial change in its dielectric constant, thereby changing its capacitive reactance and impedance at the frequency of the circular electric mode to which it is coupled. Thus the ferroelectric member operates as an RF. switch switching between a high impedance and a low impedance state in response to the applied bias potential.

What is claimed is:

1. An electronically tunable microwave coaxial magnetron including:

a cavity resonator dimensioned and arranged to support a circular electric mode of resonance,

a curved array of discrete reactive loading elements coupled to said circular electric mode of said cavity resonator, each of said discrete reactive loading elements having a reactance which is selectively variable between a minimum reactance and a maximum reactance,

means for exciting said cavity with microwave energy in said circular electric mode,

means for electronically varying said reactance of respective individual ones of said loading elements for tuning the resonant frequency of said cavity resonator in a stepwise fashion.

2. The apparatus of claim 1 including, means for generating a curved stream of electrons, a curved microwave interaction circuit coupled in electromagnetic wave energy exchanging relation with said stream of electrons for exciting wave energy on said microwave interaction circuit, and means for coupling said microwave interaction circuit in microwave energy exchanging relation with the circular electric mode of said circular electric mode cavity for stabilizing the operating frequency of wave energy of said microwave interaction circuit to the resonant frequency of said cavity resonator, whereby electronically varying the reactive loading effect of respective ones of said reactive loading elements on said cavity resonator structure tunes the frequency of wave energy coupled onto said microwave interaction circuit.

3. The apparatus of claim 2 wherein said means for generating a curved stream of electrons comprises a cathode emitter coaxially disposed of said microwave interaction circuit means, means for applying an electrical potential between said cathode and said microwave interaction circuit, and including means for generating an axial magnetic field in the circular stream of electrons for producing a crossed electric and magnetic field mode of electronic interaction between the electron stream and said microwave interaction circuit.

4. The apparatus of claim 1 wherein said reactive loading elements are disposed within said circular electric mode cavity resonator, and wherein said means for varying the reactive loading reactance of respective ones of said reactive loading elements includes, tuning means responsive to an electrical or magnetic signal and being disposed in electromagnetic field exchanging relation with respective ones of said reactive loading elements for varying the reactive loading effect of respective ones of said reactive loading elements on said cavity resonator in response to an applied electric or magnetic signal for tuning said cavity resonator structure.

5. The apparatus of claim 4 wherein said reactive loading elements comprise a set of conductive vanes periodically spaced in the direction of the circular electric field, said vanes projecting into said cavity resonator from a wall thereof.

6. The apparatus of claim 5 wherein said vanes are disposed perpendicular to said circular electric field.

7. The apparatus of claim 4 wherein said tuning means includes, diode means for electronically shorting the electric fields of respective ones of said reactive loading elements in accordance with an electrical signal applied across said diode means.

8. The apparatus of claim 4 wherein said tuning means includes, a ferromagnetic means for varying the inductive reactance at respective ones of said reactive loading elements in accordance with a magnetic signal applied to respective ones of said ferromagnetic means.

9. The apparatus of claim 4 wherein said tuning means includes, diode means for varying the connection of respective ones of said reactive loading ele- 10 ments with the circulating currents of said cavity resonator.

10. The apparatus of claim 4 wherein said reactive loading elements each include a pair of conductive vanes projecting into said cavity resonator from a wall thereof.

11. The apparatus of claim 4 wherein said reactive loading elements each includes a pair of spaced conductive members to define a gap therebetween and diode means connected across said gap for providing an electrical connection across said gap in response to forward bias applied to said diode mean.

12. The apparatus of claim 4 wherein said reactive loading elements each includes a pair of segments of a conductive ring, and diode means connected between adjacent segments of said ring for electrically connecting together adjacent ones of said segments upon biasing of said diode means into the forward conductive region.

13. The apparatus of claim 4 wherein said discrete reactive loading elements each includes a member made of a material having a dielectric constant which is selectively variable in response to an electrical bias applied thereto, whereby the capacitive reactance of said reactive loading may be selectively varied.

14. The apparatus according to claim 13 wherein said material is a ferroelectric ceramic.

15. The apparatus according to claim 14 wherein said ferroelectric ceramic comprises barium titanate. 

1. An electronically tunable microwave coaxial magnetron including: a cavity resonator dimensioned and arranged to support a circular electric mode of resonance, a curved array of discrete reactive loading elements coupled to said circular electric mode of said cavity resonator, each of said discrete reactive loading elements having a reactance which is selectively variable between a minimum reactance and a maximum reactance, means for exciting Said cavity with microwave energy in said circular electric mode, means for electronically varying said reactance of respective individual ones of said loading elements for tuning the resonant frequency of said cavity resonator in a stepwise fashion.
 2. The apparatus of claim 1 including, means for generating a curved stream of electrons, a curved microwave interaction circuit coupled in electromagnetic wave energy exchanging relation with said stream of electrons for exciting wave energy on said microwave interaction circuit, and means for coupling said microwave interaction circuit in microwave energy exchanging relation with the circular electric mode of said circular electric mode cavity for stabilizing the operating frequency of wave energy of said microwave interaction circuit to the resonant frequency of said cavity resonator, whereby electronically varying the reactive loading effect of respective ones of said reactive loading elements on said cavity resonator structure tunes the frequency of wave energy coupled onto said microwave interaction circuit.
 3. The apparatus of claim 2 wherein said means for generating a curved stream of electrons comprises a cathode emitter coaxially disposed of said microwave interaction circuit means, means for applying an electrical potential between said cathode and said microwave interaction circuit, and including means for generating an axial magnetic field in the circular stream of electrons for producing a crossed electric and magnetic field mode of electronic interaction between the electron stream and said microwave interaction circuit.
 4. The apparatus of claim 1 wherein said reactive loading elements are disposed within said circular electric mode cavity resonator, and wherein said means for varying the reactive loading reactance of respective ones of said reactive loading elements includes, tuning means responsive to an electrical or magnetic signal and being disposed in electromagnetic field exchanging relation with respective ones of said reactive loading elements for varying the reactive loading effect of respective ones of said reactive loading elements on said cavity resonator in response to an applied electric or magnetic signal for tuning said cavity resonator structure.
 5. The apparatus of claim 4 wherein said reactive loading elements comprise a set of conductive vanes periodically spaced in the direction of the circular electric field, said vanes projecting into said cavity resonator from a wall thereof.
 6. The apparatus of claim 5 wherein said vanes are disposed perpendicular to said circular electric field.
 7. The apparatus of claim 4 wherein said tuning means includes, diode means for electronically shorting the electric fields of respective ones of said reactive loading elements in accordance with an electrical signal applied across said diode means.
 8. The apparatus of claim 4 wherein said tuning means includes, a ferromagnetic means for varying the inductive reactance at respective ones of said reactive loading elements in accordance with a magnetic signal applied to respective ones of said ferromagnetic means.
 9. The apparatus of claim 4 wherein said tuning means includes, diode means for varying the connection of respective ones of said reactive loading elements with the circulating currents of said cavity resonator.
 10. The apparatus of claim 4 wherein said reactive loading elements each include a pair of conductive vanes projecting into said cavity resonator from a wall thereof.
 11. The apparatus of claim 4 wherein said reactive loading elements each includes a pair of spaced conductive members to define a gap therebetween and diode means connected across said gap for providing an electrical connection across said gap in response to forward bias applied to said diode mean.
 12. The apparatus of claim 4 wherein said reactive loading elements each includes a pair of segments of a conductive ring, and diode means connected between adjacent segments of said ring for Electrically connecting together adjacent ones of said segments upon biasing of said diode means into the forward conductive region.
 13. The apparatus of claim 4 wherein said discrete reactive loading elements each includes a member made of a material having a dielectric constant which is selectively variable in response to an electrical bias applied thereto, whereby the capacitive reactance of said reactive loading may be selectively varied.
 14. The apparatus according to claim 13 wherein said material is a ferroelectric ceramic.
 15. The apparatus according to claim 14 wherein said ferroelectric ceramic comprises barium titanate. 